This invention relates to an apparatus for moving an element in an opto-mechanical system, and in particular for example to an apparatus for moving a lens in a lens focussing operation, for example for use in a wire bonding machine.
Wire bonding machines and other apparatus used in the semiconductor manufacturing and assembly industry, often use a computerised vision system, for example for monitoring processes and for precise and accurate location of the position and orientation of objects such as semiconductor devices. Such vision systems include an opto-mechanical system for, in particular, moving optical elements such as a lens for the purpose of focussing operations.
Flexures are routinely used in opto-mechanical systems for manipulation of optical elements such as a mirror or a lens in focussing and tracking (U.S. Pat. Nos. 4,927,235, 5,208,703, 5,594,820, 6,064,505). Flexures are especially suited for these purposes due to the excellent inherent repeatability of their motion trajectory devoid of friction and wear. In many applications such as, for example, disc drives, a pair of flat parallel flexures similar to the ones shown in FIG. 1, are used. They closely mimic the classic four bar linkage. Thus, while the free end of each flexure moves along an arc, the rigid coupling between the flexures ensures that there is virtually no rotation at the free end. For a small displacement xcex94x, this approximates linear motion with a tolerably small parasitic motion xcex94z, perpendicular to the direction of the gross motion, but with virtually no lens tilt.
However, in some applications, such as in a computerized vision system of a wire bonding machine, both the orientation and the position of the optical axis of the moving lens have to be maintained within tight tolerances, over the entire range of lens motion. In such cases, parallel flexures mentioned above are not suitable. Flexures with circular symmetry do not suffer from parasitic lateral motion but instead, give rise to a small angular rotation about the motion axis. Such flexures have been used in long life compressors and cryogenic coolers as disclosed in U.S. Pat. Nos. 5,351,490, 5,492,313, 5,522,214, 5,647,217, 5,920,133, 6,050,556 and 6,129,527. Some typical designs are shown in FIG. 2. These are in the form of flat discs, fractions of a millimeter thick. Each disc has a specified number of slots (usually but not always, three), of either spiral or straight or arc shape or a combination thereof. They are machined using either wire electro-discharge machining or photo-lithography or any other suitable method, yielding a number of flexing xe2x80x9carmsxe2x80x9d which bear the load of the moving member. Very high ratios of radial stiffness to axial stiffness can be realized using such flexures.
Actuation of the mechanism using flexures of circular symmetry is normally effected by a brush-less linear motor using permanent magnets. The linear motor could be either of single phase (commonly called voice coil motor) or of multiphase design, in any of the several possible topologies but most usually cylindrical. While making best use of the available space, leading to a very compact design, the cylindrical voice coil motor keeps the actuating force virtually aligned with the axis of the flexure discs. A position sensor provides position feedback enabling the motor to be operated in closed loop servo mode for very precise control over the axial position of the optical component mounted on the moving member.
A problem associated with flexure bearings is the occurrence of self-excited vibrations at the end of the stroke. In the absence of sufficient damping in the system, these vibrations continue for an undesirably long period of time before their attenuation reaches a level acceptable to the specified objective of the motion stage, such as in the present case, moving a lens to focus an image of interest. This considerably slows down the entire process. In order to substantially speed up the attenuation of undesirable vibrations some kind of damping needs to be introduced into the system.
According to the present invention there is provided apparatus for moving an optical element in an opto-mechanical system, comprising at least one circularly symmetric flexure surrounding said optical element, and means for moving said optical element along an axis perpendicular to and extending through the centre of said at least one flexure.
Preferably there may be a stack of circularly symmetric flexures, and preferably the flexures in each said stack may be separated by spacer members. Such spacer members may include rim spacers for spacing apart rim portions of said flexures, and central spacers for spacing apart central portions of said spacers.
In a particularly preferred embodiment of the invention the optical element is mounted between two stacks of flexures and the axis of movement of the optical element extends perpendicular to and through the centre of each stack.
The means for moving the optical element may comprise a voice coil motor.
Means may be provided for sensing the position of said optical element and for providing a feedback control to said moving means. The sensing means may comprise a linear variable differential transducer, an optical sensor, a capacitive sensor or an inductive sensor.
Another preferred feature of the invention is that the optical element may be supported within a tubular member that moves with the optical element, such that an end of the tubular member is received within a bore formed as part of a fixed component, the bore having an internal diameter slightly greater than the external diameter of the tubular member to define a narrow annular passage therebetween, whereby movement of the optical element may be damped by the movement of air into and out of the bore through the annular passage.